Task manager + delegating the current thread to background

[mahrud]

As an addendum to #3454, it would be fantastic if we had a "task manager" for M2. Here are the features I'd like to see first:

1. list all tasks, including created, running, finished, cancelled, and terminated tasks and some information like how long they have been running
2. connect stdio/stderr to a task to see its output
3. bring a task to foreground so that ctrl-c lands me in its debugger
4. retrieve the output of a finished task
5. cancel a runaway task

In the future, it would be great to also have the ability to enable/disable profiling on a thread after it has started to get some insight into what it is doing.

Additionally, one feature that I'm surprised I haven't seen before:

• if a running function in the current thread is taking too long, it would be great to delegate it to a background task, perhaps by a keyboard shortcut (ctrl-z?), so that it can be retrieved later.